of oxygen and
for its increased rate of discharge to the tissues, the most important
features of which are a diminution in the size of the corpuscle and the
attainment of its peculiar shape, both resulting in the production of a
relatively enormous corpuscular surface in a unit volume of blood.

Variations are also found in the white corpuscles as well as in the red,
but these differences are not so striking and lie chiefly in unimportant
details of structure of individual cells. Enormous variations are to be
found in different species of mammals, but the cells generally conform
to the types of secreting cells or phagocytes.

The platelets also differ in the different species. In the frog, for
instance, many are spindle-shaped and contain a nucleus-like structure.
Birds' blood is stated to contain no platelets. The variations in number
of these bodies have not been satisfactorily ascertained on account of
the difficulties involved in any attempt to preserve them and to render
them visible under the microscope.

Differences are also found in the chemical composition of the plasma.
The chief variation is in the amount of protein present, which attains
its maximum concentration in birds and mammals, while in reptiles,
amphibia and fishes it is much less. The bloods of the latter two
classes are much more watery than that of the mammal. Moreover, it has
been proved that there are specific differences in the chemical nature
of the various proteins present even between different varieties of
mammals. Thus the ratio of the globulin fraction to the albumin fraction
may vary considerably, and again, one or other of the proteins may be
quite specific for the animal from which it is derived.

_Clotting._--If a sample of blood be withdrawn from an animal, within a
short time it undergoes a series of changes and becomes converted into a
stiff jelly. It is said to _clot_. If the process is watched it is seen
to start first from the surfaces where it is in contact with any foreign
body; thence it extends through the blood until the whole mass sets
solid. A short time elapses before this process commences--a time
dependent upon two chief conditions, viz. the temperature at which the
blood is kept and the extent of foreign surface with which it is brought
into contact. Thus in a mammal the blood clots most quickly at a
temperature a little above body temperature, while if the blood be
cooled quickly the clotting is considerably delay